1. Field of the Invention
The present invention relates to a polarization independent semiconductor optical amplifier with high saturation input and high saturation gain.
2. Prior Art
Recently, the optical communication technology has been progressing as one of the technologies for a large capacity and high speed transmission. Amongst various optical communication technologies, a switching function of the semiconductor optical amplifiers as well as an amplifier function is of great interest, because the optical amplifier absorbs light without injection current.
Preferably, the optical amplifier is expected to amplify signal light which passes through an optical fiber without controlling the polarization. Accordingly, the polarization independent gain is desirable.
Therefore, various methods for implementing the polarization independent gain have been proposed. An example of the polarization independent semiconductor optical amplifier is reported in IEEE PHOTONICS TECHNOLOGY LETTERS(VOL.6, NO.2, FEBRUARY 1994, p170 to p172: P. Doussiere ). The polarization independent amplifier described in the above-mentioned document is compatible with the single mode output of light. Polarization independence is achieved by preparing a nearly square shaped active layer constructed by a bulk crystal, while the single mode condition is satisfied by narrowing the active layer to the grade of 0.5 .mu.m.
In the above-mentioned reference, the waveguide is fabricated as narrow as about 0.5 .mu.m in order to achieve both the polarization independent gain and the single mode wave output. However, it is not easy to fabricate accurately such an extremely narrow waveguide on a semiconductor substrate, with good reproduction and mass production yield.